1. Field of the Invention
The present invention relates to a mechanical fuse for disconnecting power transmission when an overload occurs in a power transmission device to ensure the safety of the device, and relates to a production method for the same.
2. Description of the Related Art
A mechanical fuse is provided, for example, in a power transmission device interposed between a driving side and a driven side (for example, between a driving shaft and a driven shaft) to transmit the rotational power of the driving side to the driven side. The fuse forces the disconnecting of power transmission when a large difference occurs between the driving side and the driven side during transmission of rotating power, thereby avoiding occurrence of lock-up or rupture due to propagation of overload torque to one side.
Such a mechanical fuse is generally composed of a rupturable member which sacrificially ruptures, disposed between two holding members held to the driving side and driven side so as to link these holding members. The rupturable member may specifically be a shear pin or a shear plate, which is required to rupture reliably when a specified torque or more is applied. In addition, the mechanical fuse may be embodied so as to unify the above two holding members, for example, an embodiment in which a boss portion is connected to the driving side (or the driven side) by forming a screw thread inside (or outside) thereof and a flange portion is connected to the driven side (or the driving side) by forming a screw thread outside (or inside) thereof. In an embodied connection of the boss portion and the flange portion, a difference portion corresponds to the above rupturable member, and the power transmission is forced to be disconnected by breaking the difference portion when a large differential operation occurs between the driving side and the driven side. Furthermore, in the embodiment, the screw phase of the driving side and that of the driven side may be changed to be the opposite phase from each other by selecting a rotational direction in which the screw tightens (torque affects the differential portion).
The magnitude of the torque which will cause rupture can be adjusted by selection of the material and the cross sectional area of the rupturable member; however, if the rupturable member is made of an unstable material which may be reduced in strength by fatigue due to repeated exposures to load, the reliability thereof is low, and it is therefore not practical. As a material to satisfy the requirements, hitherto, ceramics have been widely used because of stability in fatigue strength and high fatigue limit ratio (ratio of fatigue strength to tensile strength, determined by fatigue strength÷tensile strength).
However, although ceramics are stable in fatigue strength, they are expensive, and hence, less expensive mechanical fuses have been desired. There is also a need for a mechanical fuse in the form of rupturable members integrated with holding members at both sides and interposing a rupturable portion between holding portions for the purpose of improving assembling efficiency and general applicability, but the mechanical fuse in this form is not used at present because there are problems in that it is difficult to assure the strength of holding portions and because broken pieces scatter after rupturing.
In order to solve the problems described above, the present applicants proposed a mechanical fuse composed of Fe-based sintered alloy having pores having a roundness of 0.004 or more which is controlled by forming an iron oxide layer on a pore inner wall (Japanese Patent Publication No. 2002-168267A). According to this mechanical fuse, there is an advantage in that it is inexpensive, is superior in forming properties, and is high in fatigue limit ratio.
The mechanical fuse disclosed in Japanese Patent Publication No. 2002-168267A has a matrix strengthened by adding C; however, combined carbon content changes slightly due to effects of the sintering atmosphere, and fatigue limit ratio thereby varies somewhat. Therefore, in order to produce a fuse so that the fatigue limit ratio is stable, it is necessary to take measures in which the sintering atmosphere is strictly controlled, or the like.
In addition, since mechanical fuses have been recently used in environments in which high reliability is required, further improvement of the fatigue limit ratio is required, and in particular, high reliability under a use condition in which cyclic loading is loaded is required.